Surface treated steel plate for battery cases and battery case using same

ABSTRACT

A surface treated steep plate excellent in battery performance and a battery case comprising the same are disclosed. The battery case is produced by forming a surface treated steel sheet having an indium layer on the outermost side to serve as the inner surface of a battery case of a plating base steel plate by deep drawing, DI or DTR.

TECHNICAL FIELD

This invention relates to a container for holding an alkaline solution,and more particularly to a surface treated steel sheet for batterycases, such as for alkaline manganese or nickel-cadmium batteries, and abattery case formed therefrom by the deep drawing, DI or DTR method.

BACKGROUND ART

Battery cases for holding a strongly alkaline solution, such as foralkaline manganese or nickel-cadmium batteries, have hitherto been madeby press forming a battery case from a cold rolled steel strip andbarrel plating it, or by press forming a battery case from anickel-plated steel strip.

The use of nickel plating for batteries, such as alkaline manganese ornickel-cadmium batteries, as stated, is due to the high resistance ofnickel to alkaline corrosion, as those batteries use mainly stronglyalkaline potassium hydroxide as the electrolyte, also to the constantcontact resistance of nickel which is important when the batteries areconnected to external terminals, and moreover to the high spotweldability pf nickel, as the manufacture of batteries relies upon spotwelding to weld the component parts together into batteries.

The DI (drawing and ironing) method has recently come to be employed asa method of press forming a battery case with a thin wall to achieve anincreased battery capacity, instead of the deep drawing method (JapanesePatent Office Official Gazette JP-B-H7-99686). The DI or DTR (drawingthin and redraw) method can advantageously form a case having a sidewallthickness smaller than its bottom thickness and a correspondinglygreater space for holding the active substances for the positive andnegative electrodes and thereby making it possible to obtain a batteryof increased capacity, while its large bottom thickness gives thebattery an improved pressure resistance.

Moreover, the alkaline manganese batteries have recently come to berequired to show an outstanding performance in internal resistance,short-circuit current, discharge characteristics, etc.

The battery cases formed by the deep drawing, DI or DTR method haverecently come to have an inner surface layer formed by a nickel platinglayer or an iron-nickel diffused layer to achieve an improved batteryperformance, as stated above.

The can having a nickel plating layer or an iron-nickel diffused layeras the outermost layer of its inner surface restricts the batterycharacteristics and some improvement is desired.

It is an object of this invention to provide a battery case achieving anoutstanding battery performance and a surface treated steel sheet whichis suitable for use in manufacturing such a battery case.

DISCLOSURE OF THE INVENTION

Under these circumstances, we, the inventors of this invention, havefound that a battery case formed by the deep drawing, DI or DTR methodprovides an outstanding battery performance in internal resistance,short-circuit current, etc. if it has an indium layer as the outermostlayer of its inner surface.

In order to attain the above object, a surface treated steel sheet forbattery cases as set forth in claim 1 is characterized by having anindium layer on at least one surface thereof. The indium layer ispreferably formed by electrolytic plating and is preferably formed onthe steel surface supposed to form the inner surface of a battery case.

The steel sheet preferably has a nickel or nickel alloy layer formed asa lower layer on its surface supposed to form the inner surface of abattery case and an indium layer formed as an upper layer thereon. Thenickel alloy layer preferably contains one or more of a nickel-tinalloy, a nickel-iron alloy, a nickel-iron diffused layer, anickel-phosphorus alloy and a nickel-cobalt alloy.

Moreover, the steel sheet preferably has an iron-nickel diffused layerformed as a lower layer on its surface supposed to form the innersurface of a battery case, a nickel layer as a middle layer and anindium layer as an upper layer.

A battery case as set forth in claim 7 is characterized by being formedby the deep drawing, DI or DTR method from a surface treated steel sheetfor battery cases as set forth in any of claims 1 to 6.

BEST MODE OF CARRYING OUT THE INVENTION

The invention will now be described progressively.

(1) Steel Sheet:

A mild steel sheet is first prepared for manufacturing a battery caseaccording to this invention. The mild steel sheet is preferably of, forexample, cold rolled Al-killed low-carbon steel, very low-carbon steelhaving a carbon content of 0.003% or less, or non-aging very low-carbonsteel containing niobium, boron and titanium.

The mild steel sheet is used to facilitate the formation of a can by thedeep drawing, DI or DTR method.

(2) Nickel or Nickel Alloy Plating:

Referring to the formation of nickel or nickel alloy plating on abattery case and a surface treated steel sheet as stated above, it ispossible to use any known electroless plating or electroplating bath,such as a vat, a sulfamate bath, a borofluoride bath or a chloride bath.A plating thickness of, say, 0.5 to 3.0 μm is preferred. Any thicknessbelow 0.5 μm is objected to from the standpoint of alkali resistance andany thickness over 3.0 μm is economically undesirable. Nickel alloyplating is preferably carried out by performing electroplating afteradding a compound of iron, tin, phosphorus, cobalt, etc. to any suchknown bath. Nickel-phosphorus alloy plating may be carried out byperforming electroless plating in a known electroless plating bath, suchas phosphorous acid, phosphate, hypophosphite or hypophosphite.

(3) Diffusion by Heat Treatment:

After nickel or nickel alloy plating as described at (2) above, an alloylayer may be formed by diffusion by heat treatment. In the case of, forexample, nickel or nickel-iron alloy plating, a nickel-iron diffusedlayer is formed. If tin plating is performed after nickel plating at (2)above and is followed by heat treatment, there is formed a nickel-tindiffused layer, or two layers consisting of an iron-nickel diffusedlayer as a lower layer and a nickel-tin diffused layer as an upperlayer. The nickel-tin diffused layer is, among others, preferred for itshigh alkali resistance.

The heat treatment is preferably carried out in a non-oxidizing orreducing protective gas atmosphere to prevent the formation of any oxidefilm on the surface of the alloy layer. An inert gas, such as nitrogen,argon or neon, is suitable for use as the non-oxidizing gas, andhydrogen or ammonia gas is, for example, suitable as the reducing gas.The heat treatment may be carried out by batch or continuous annealing.A heat treating temperature of 300 to 900° C. is preferable and atreating time of, say, 30 seconds to 15 hours is preferable, but theconditions of heat treatment depend on the steel sheet used, forexample, a very low-carbon steel having a carbon content of 0.003% byweight or less requires a constant temperature and a short time, sincethe steel base has a high recrystallization temperature.

(4) Indium Plating:

After its nickel or nickel alloy plating, or after its subsequent heattreatment as described, the steel sheet is plated with indium. Indium isa metal which is of high alkali resistance and low contact resistanceand is soft, and makes, therefore, intimate contact with a positiveelectrode mixture. Although for the purpose of this invention, it issatisfactory to use for indium plating any known plating bath, such as ahigh pH cyanide bath, a sulfate bath, a borofluoride bath, a sulfamatebath, a metasulfonate bath or an NTA bath, it is often preferable to usea simple sulfate bath.

This bath contains 10 to 25 g of indium sulfate and 0 to 10 g of sodiumsulfate per liter and is used for plating at a pH of 2.0 to 2.7, at roomtemperature and at a current density of 2 to 4 A/dm² by using an indiumanode. The thickness of an indium plating layer is usually controlled byemploying a different current density.

The plating layer preferably has a thickness of, say, 50 to 500 mg/m².Its thickness below 50 mg/m² is hardly effective for realizing a lowercontact resistance and its thickness over 500 mg/m² is economicallydisadvantageous.

EXAMPLES

The invention will now be described by examples. Cold rolled, annealedand temper rolled low-carbon aluminum-killed steel sheets 0.25 and 0.4mm, respectively, in thickness were employed as the steel sheets to beplated. Cold rolled very low-carbon aluminum-killed steel sheets of 0.25mm and 0.4 mm, respectively, in thickness were also employed as thesteel sheets to be plated. The chemical composition of the two kinds ofsteel sheets was as shown below.

C: 0.04% (by weight; in every other case, too)

Si: 0.01%

Mn: 0.22%

P: 0.012%

S: 0.006%

Al: 0.0.48%

N: 0.0025%

The steel sheets to be plated were subjected to pre-treatment bycustomary methods, including alkali electrolytic degreasing, waterrinsing, sulfuric acid immersion and water rinsing and then to ordinarytreatment including non-lustrous nickel plating, as shown below.

1) Non-Lustrous Nickel Plating

Non-lustrous nickel plating was performed by using a nickel sulfatebath.

Bath composition: Nickel sulfate (NiSO₄.6H₂O) 300 g/l Nickel chloride(NiCl₂.6H₂O) 45 g/l Boric acid (H₃BO₃) 30 g/l

-   -   Bath pH: 4 (adjusted by sulfuric acid)    -   Agitation: Air agitation    -   Bath temperature: 60° C.    -   Anode: A titanium basket filled with S pellets (trade name of        product of INCO; spherical) and covered with a polypropylene        bag.

The plating bath which will be described below is used for semi-lustrousnickel plating. This semi-lustrous nickel plating may alternativelyreplace the first non-lustrous nickel plating.

2) Semi-Lustrous Nickel Plating

Semi-lustrous nickel plating was performed by adding a polyoxyethyleneaddition product of unsaturated alcohol and unsaturated carboxylic acidformaldehyde appropriately as semi-lustrous agents to a nickel sulfatebath.

Bath composition: Nickel sulfate (NiSO₄.6H₂O) 300 g/l Nickel chloride(NiCl₂.6H₂O) 45 g/l Boric acid (H₃BO₃) 30 g/l

-   -   Polyoxyethylene addition product of unsaturated alcohol 3.0 g/l    -   Unsaturated carboxylic acid formaldehyde 3.0 g/l    -   Bath pH: 4 (adjusted by sulfuric acid)    -   Agitation: Air agitation    -   Bath temperature: 60° C.    -   Anode: A titanium basket filled with S pellets (trade name of        product of INCO; spherical) and covered with a polypropylene        bag.

A nickel-tin alloy layer can be formed by diffusion treatment asdescribed later, if tin plating is performed after the non-lustrous orsemi-lustrous nickel plating.

The nickel plating described above may be replaced by nickel alloyplating.

3) Nickel Alloy Plating

Nickel alloy plating is performed by adding a known compound of analloying element, such as iron, tin, phosphorus or cobalt, to a knownnickel plating bath. For example, nickel-iron alloy plating wasperformed by adding iron sulfate appropriately to a nickel sulfate bathto have iron included in a nickel plating layer.

Bath composition: Nickel sulfate (NiSO₄.6H₂O) 320 g/l Nickel chloride(NiCl₂.6H₂O) 20 g/l Iron sulfate (FeSO₄.6H₂O) (appropriately) Boric acid(H₃BO₃) 30 g/l

-   -   Bath pH: 4 (adjusted by sulfuric acid)    -   Agitation: Air agitation    -   Bath temperature: 60° C.    -   Anode: A titanium basket filled with S pellets (trade name of        product of INCO; spherical) and covered

with a polypropylene bag. If it is desired to have phosphorus or cobaltincluded instead of iron, sodium hypophosphite or cobalt sulfate may beadded appropriately to the plating bath to replace iron sulfate.

The iron, phosphorus or cobalt contents of plating films and theirthicknesses were altered under the conditions stated above. The platingconditions are shown in Table 1. A known fluoride bath can alternativelybe used for nickel-tin alloy plating.

4) Diffusion by Heat Treatment

Heat treatment may be performed for diffusion after the nickel or nickelalloy plating described above. Heat treatment may alternatively beperformed for diffusion after the formation of two plating layers bynickel and tin plating. The diffusion treatment is preferably performedin a non-oxidizing or reducing atmosphere, for example, in anon-oxidizing atmosphere containing 6.5% of hydrogen, the rest of whichis nitrogen. A known apparatus, such as a batch or continuous annealingfurnace, may be used for the diffusion treatment.

The conditions of the heat treatment are shown in Table 1.

5) Indium Plating

Indium plating was formed under the following conditions:

Bath composition: Indium sulfate: 10 to 25 g/l Sodium sulfate: 0 to 10g/l

pH: 2.0 to 2.7

Anode: Indium

Bath temperature: Room temperature

Current density: 2 to 4 A/dm²

The thickness of an indium plating layer is usually controlled byaltering the current density, and is shown in Table 1.

(Forming a Battery Case)

The formation of a battery case by the DI method was carried out byusing the plated steel sheet having a thickness of 0.4 mm, shaping ablank having a diameter of 41 mm into a cup having a diameter of 20.5 mmand subjecting it to redrawing and two stages of ironing by a DI machineto form a case having an outside diameter of 13.8 mm, a wall thicknessof 0.20 mm and a height of 56 mm. It was finally trimmed at its top togive an LR6 battery case having a height of 49.3 mm. The DI method wasapplied to the surface treated steel sheets according to Examples 1 to 3and Comparative Example 1.

The formation of a battery case by the DTR method was carried out byusing a plated steel sheet having a thickness of 0.25 mm, punching itinto a blank having a diameter of 58 mm and subjecting it to severaltimes of drawing and redrawing to form an LR6 battery case having anoutside diameter of 13.8 mm, a wall thickness of 0.20 mm and a height of49.3 mm. The DTR method was applied to the surface treated steel sheetsaccording to Examples 4 to 6 and Comparative Example 2.

The formation of a battery case by the deep drawing method was carriedout by using a plated steel sheet having a thickness of 0.25 mm,punching it into a blank having a diameter of 57 mm and subjecting it toseveral times of drawing and redrawing to form an LR6 battery casehaving an outside diameter of 13.8 mm, a wall thickness of 0.25 mm and aheight of 49.3 mm. The deep drawing method was applied to the surfacetreated steel sheets according to Examples 7 and 8 and ComparativeExample 3. TABLE 1 Conditions for Sample Manufacture and BatteryPerformance Example or Inner or outer Undercoat plating Heat treatmentIndium plating Battery performance Comparative surface of Plating weightafter undercoat weight IR SCC Discharge at Example battery case Kind ofplating (g/m²) plating (mg/m²) (m Ω) (A) 1 A (min.) Example 1 Innersurface Non-lustrous Ni 8.7 None 52 132 8.1 15.8 Outer surfaceNon-lustrous Ni 17.8 — Example 2 Inner surface Semi-lustrous Ni 17.5None 109 138 7.9 16.1 Outer surface Semi-lustrous Ni 18.2 — Example 3Inner surface Non-lustrous Ni 17.8 None 256 133 7.9 16.1 Outer surfaceSemi-lustrous Ni 17.8 — Example 4 Inner surface Non-lustrous Ni 17.6None 486 129 8.4 17.0 Outer surface Semi-lustrous Ni 18.3 — Example 5Inner surface Ni-3% wt P 7.7 550° C. × 8 h 55 138 7.8 15.9 Outer surfaceNi-3% wt P 17.9 — Example 6 Inner surface Non-lustrous Ni 17.9 550° C. ×8 h 316 135 8.0 16.8 Outer surface Semi-lustrous Ni 17.6 — Example 7Inner surface Ni-5 wt % Co 18.1 780° C. × 2 min 402 134 8.2 17.0 Outersurface Ni-5 wt % Co 17.5 — Example 8 Inner surface Ni-3 wt % Fe 17.7780° C. × 2 min 496 131 8.4 17.2 Outer surface Ni-3 wt % Fe 17.8 —Comparative Inner surface Non-lustrous Ni 8.8 None — 159 6.4 14.0Example 1 Outer surface Non-lustrous Ni 18.5 — Comparative Inner surfaceNon-lustrous Ni 17.9 550° C. × 8 h — 164 5.7 13.8 Example 2 Outersurface Non-lustrous Ni 17.6 — Comparative Inner surface Semi-lustrousNi 18.2 780° C. × 2 min — 165 6 13.6 Example 3 Outer surfaceSemi-lustrous Ni 18.1 —

The battery cases as formed above were used for manufacturing alkalinemanganese batteries and they were evaluated for characteristics. Theresults of their evaluation are shown in Table 1.

[Evaluation for Internal Resistance (IR)]

Each battery as manufactured was examined for its internal resistance(mΩ) by the AC impedance method after three days of storage at 80° C.

[Evaluation for Short-Circuit Current (SCC)]

After three days of storage at 80° C., an ammeter was connected to eachbattery as manufactured to form a closed circuit and the current valueof the battery was measured as its short-circuit current.

[Discharge Characteristics]

After three days of storage at 80° C., a resistance of 2 Ω was connectedto each battery as manufactured to form a closed circuit and time wasmeasured until a voltage of 0.9 V was reached.

INDUSTRIAL APPLICABILITY

As is obvious from Table 1, the alkaline manganese batteries made byusing for the positive electrode plate a steel sheet having an indiumlayer formed on its surface forming the inner surface of the batterycase have a low internal resistance and a high short-circuit current ascompared with any known alkaline manganese dry battery having a nickelplating or iron-nickel diffused layer on its surface, and also differsignificantly from any such known alkaline manganese dry battery in theduration of continuous discharge.

1. A surface treated steel sheet for battery cases having an indiumlayer at least on one surface thereof.
 2. A surface treated steel sheetfor battery cases as set forth in claim 1, wherein the indium layer isformed by electrolytic plating.
 3. A surface treated steel sheet forbattery cases as set forth in claim 1, wherein the indium layer isformed on the steel surface supposed to form the inner surface of abattery case.
 4. A surface treated steel sheet for battery cases as setforth in claim 1, wherein the steel sheet has a nickel or nickel alloylayer formed as a lower layer on its surface supposed to form the innersurface of a battery case and an indium layer formed as an upper layerthereon.
 5. A surface treated steel sheet for battery cases as set forthin claim 4, wherein the nickel alloy layer contains one or more of anickel-tin alloy, a nickel-iron alloy, a nickel-iron diffused layer, anickel-phosphorus alloy and a nickel-cobalt alloy.
 6. A surface treatedsteel sheet for battery cases as set forth in claim 1, wherein the steelsheet has an iron-nickel diffused layer formed as a lower layer on itssurface supposed to form the inner surface of a battery case, a nickellayer as a middle layer and an indium layer as an upper layer.
 7. Abattery case formed by the deep drawing, DI or DTR method from a surfacetreated steel sheet for battery cases as set forth in claim 1.